1. Field of the Invention
The present invention relates to a piezoelectric transformer inverter, and more particularly to a piezoelectric transformer inverter for lighting a cold cathode fluorescent tube used in a liquid crystal display panel of a notebook-size personal computer or the like.
2. Description of the Related Art
Examples of a piezoelectric transformer inverter used for lighting a cold cathode fluorescent tube in a liquid crystal back-lighting have been described in Japanese Unexamined Patent Publication No. 6-167694 and Japanese Unexamined Patent Publication No. 9-107684. In Japanese Unexamined Patent Publication No. 6-167694, luminance of the cold cathode fluorescent tube is adjusted by controlling tube current of the cold cathode fluorescent tube used for back-lighting. In this case, by using the frequency-step-up ratio characteristics of a piezoelectric transformer, the tube current can be controlled. For example, when a small amount of the tube current is required, or the input voltage is high, control is such that a certain amount of tube current can be obtained by changing an inverter frequency into a frequency higher than a resonance frequency of the piezoelectric transformer.
In this system, however, when the input voltage is high, the inverter frequency is significantly deviated from the resonance frequency of the piezoelectric transformer. In a piezoelectric transformer, the highest conversion efficiency is gained when the frequency is slightly within the high frequency side of the resonance frequency. In contrast, when the frequency is deviated from this position, the conversion efficiency is lowered. As a result, in this system, the higher the input voltage, the greater the degree of the efficiency reduction, whereby only a range of approximately twice the input voltage is applicable for practical use.
In addition, regarding a piezoelectric-transformer drive circuit described in Japanese Unexamined Patent Publication No. 9-107684, a chopper circuit is inserted in the front stage of an inverter unit so as to maintain an average input voltage which is applied into the inverter unit constant. This arrangement permits the aforementioned problems of the conventional art to be solved, whereby appropriate efficiency can be maintained even in a wider range of input voltage.
In this piezoelectric-transformer drive circuit, however, there are problems as described below, when it is used in a notebook-size personal computer or the like. For example, when the input voltage applied into an inverter is set to 7 through 20V, as represented by a waveform A in FIG. 5, and the average voltage of output of a chopper circuit is set to 6.5V, there is provided an assumption that first, when the notebook-size personal computer is battery-driven, the input voltage is set to 7V as the worst condition. In this case, the duty of the chopper-circuit output is, as represented by a waveform B in FIG. 5, indicated by the formula: D=6.5/7=93%. Now, in this state, it is assumed that an AC adapter is inserted, thereby input voltage rises up to 20V as the worst condition. At this moment, the duty of output of the chopper circuit needs to be changed as follows: EQU D=6.5/20=33%.
However, since a feedback control is given to maintain output voltage of the duty of the chopper circuit constant, there is a time lag in the control response. As a result, in fact, as represented by a waveform C in FIG. 5, control release occurs for some time after an abrupt change in the input voltage, whereby the inverter average output voltage increases transitionally. At this moment, the drain voltage of an FET of the inverter unit is higher than that in a normal condition, as represented by a waveform D in FIG. 5. Consequently, in order to be tolerant of higher transitional input voltages, it is necessary to use an FET in the inverter unit which can withstand such higher voltages, so that this leads to the disadvantage of larger size.
FIG. 6 is a block diagram showing a piezoelectric transformer inverter for preventing the above-described transitional phenomenon. In order to avoid such a phenomenon, the piezoelectric transformer inverter shown in FIG. 6 can be considered. In the piezoelectric transformer inverter shown in FIG. 6, without inserting the chopper as described above, a DC-DC converter control unit 13 is inserted in the front stage of an inverter unit 12 for driving a cold cathode fluorescent tube 11. The DC--DC converter control unit 13 controls a switching transistor 16, and the switching transistor 16 controls voltage which is input in the inverter unit 12.
In addition, a diode 17 is a free wheeling device, which is connected between the output electrode of the switching transistor 16 and a ground. In the example shown in FIG. 6, a coil 14 is required to be inserted between the switching transistor 16 and the inverter unit 12, and a capacitor 15 is required to be inserted between the input side of the inverter unit 12 and a ground. Furthermore, a loss due to current flowing into the coil 14 is large and there are no advantages in terms of size and efficiency, compared with the method of inserting a chopper.